1. Field of the Invention
The present invention relates to an improved catalyst for use in the ammoxidation of an unsaturated hydrocarbon to the corresponding unsaturated nitrile. In particular, the present invention is directed to the process for making an improved catalyst for the ammoxidation of propylene and/or isobutylene to acrylonitrile and/or methacrylonitrile, respectively and the oxidation of propylene and/or isobutylene to acrolein/acrylic acid and/or methacrolein/methacrylic acid, respectively. More specifically, the invention relates to the process for producing an improved multi-component ammoxidation catalyst comprising a complex of metal oxides wherein a heat-decomposable nitrogen containing compound is added during the process for the preparation of the catalyst prior to the calcination of the catalyst.
2. Description of the Prior Art
Catalysts containing oxides of iron, bismuth and molybdenum, promoted with suitable elements, have long been used for the conversion of propylene and/or isobutylene at elevated temperatures in the presence of ammonia and oxygen (usually in the form of air) to manufacture acrylonitrile and/or methacrylonitrile. In particular, Great Britain Patent 1436475; U.S. Pat. Nos. 4,766,232; 4,377,534; 4,040,978; 4,168,246; 5,223,469 and 4,863,891 are each directed to bismuth-molybdenum-iron catalysts which may be promoted with the Group II elements to produce acrylonitrile. In addition, U.S. Pat. Nos. 5,093,299, 5,212,137, 5,658,842 and 5,834,394 are directed to bismuth-molybdenum promoted catalysts exhibiting high yields to acrylonitrile.
In part, the instant invention relates to the preparation of bismuth-molybdenum-iron catalysts and the use of an additive as part of the preparation. Typically, such catalysts are produced in a batch process by simply combining and reacting, source compounds for the various metal components. However, more complex and multiple-step preparations have been used. For example, U.S. Pat. No. 4,040,978 taught a process for catalyst preparation where molybdates of each metal were separately made and then combined and reacted; and U.S. Pat. No. 4,148,757 taught a process for catalyst preparation where bismuth and molybdenum were first reacted to form a bismuth molybdate and then the bismuth molybdate was combined with a mixture of source compounds for the various other metal components.
Other than the addition of promoter elements and the use of various support materials, such as silica, the addition of additives to promote iron bismuth molybdate ammoxidation catalysts is atypical. This is less true with other catalyst systems. For example, U.S. Pat. No. 5,128,114 teaches the addition of ammonium citrate or urea to an aqueous silica sol, drying the mixture, and then calcining the dried powder to remove volatile components. The use of the additive produces attrition resistant microspheroidal particles. U.S. Pat. No. 5,128,114 describes using this method to prepare palladium and platinum-palladium catalysts supported on silica suitable as catalysts for the production of hydrogen peroxide by reaction of hydrogen and oxygen. U.S. Pat. No. 7,288,669 and U.S. Pat. No. 6,943,135 teach adding a source of NOx to the other ingredients employed in the catalyst preparation for catalysts for propane oxidation to acrylic acid or propane ammoxidation to acrylonitrile (Mo—V—Te—Nb—O type catalyst systems are exemplified).